The need for making a non-rectifying (ohmic) contact to a semiconductor body exists throughout the semiconductor field. In the field of III/V-based semiconductors it has been conventional to use so-called alloyed contacts.
For instance, in order to make contact to the p-side of a InP-based laser diode or other 2-terminal device, it is conventional to evaporate onto p-doped semiconductor material a gold-based layer such as AuBe or AuZn (the use of "AuBe" and similar terms is conventional and does not imply equal amounts of Au and Be), followed by heating of the combination in a tube furnace for several minutes to cause alloying of the Au with the semiconductor surface and diffusion of Be (or Zn) into the underlying semiconductor material. Subsequent to this heat treatment a layer of barrier metal (Ti or Cr, followed by Pt or Pd) is typically deposited onto the alloyed layer, and the combination typically is again heat treated. This is typically followed by deposition of a relatively thick contact layer of gold, to permit wire bond attachment, or formation of a metallurgical solder bond to a Au-coated heat sink.
A similar process is conventionally used to provide a non-rectifying contact to the n-side of the laser or other two terminal device, with the principal differences being that the gold-based layer typically is AuGe, and that the alloying heat treatment is typically carried out at a somewhat lower temperature.
As can be seen from the above description, the conventional process is relatively complicated, typically requiring at least two different heat treatments, as well as typically, three metallizations per side. The resulting alloyed contacts typically are thermally stable only at relatively low temperatures, exemplarily less than about 320.degree. C. Furthermore, there exists in the conventional process a significant potential for operator error due to the non-symmetrical nature of the alloying deposits. Also, conventional processing typically comprises thinning of the semiconductor body, which typically is carried out prior to the deposition of the n-side contact. The body (exemplarily a InP wafer which is later to be divided into a multiplicity of individual devices) is lapped or otherwise thinned to reduce its thickness from about 250 .mu.m to about 80 .mu.m, resulting in a very fragile body that can very easily be broken by insufficient care in handling.
Conventional alloyed contacts themselves are frequently subject to still other shortcomings. For instance, the alloying process can result in localized Au and Be (or Zn) penetration into the semiconductor material. Such penetration "spikes" may be associated with the occurrence of so-called "dark" defects in lasers or LEDs, and may also affect the long-term reliability of the devices.
In view of the shortcomings associated with conventional alloy contacts, alternative methods of forming non-rectifying contacts have been under investigation for some time, and non-alloyed ohmic contacts to III/V-based and other semiconductor bodies are known. Such contacts can be formed if an appropriate metal is deposited onto a heavily doped semiconductor. See, for instance, A. Y. C. Yu, Solid State Electronics, Vol. 13, pp. 239-247, reporting on electron tunneling and contact resistance of metal/silicon contact barriers.
A highly desirable characteristic of an ohmic contact is a relatively low specific contact resistance. It is known that there are two ways to achieve low specific contact resistance ohmic contacts. One is to form a very thin Schottky barrier (such that tunneling readily occurs) by heavily doping the semiconductor, and the other is to utilize materials with narrow energy band gap which result in low Schottky barrier height between the semiconductor body and the contact metal.
J. M. Woodall et al., Journal of Vacuum Science and Technology, Vol. 19 (3), pp. 626-627, have studied ohmic contacts on compositionally graded n-type Ga.sub.1-x In.sub.x As structures. These structures utilize the fact that for InAs surfaces Fermi level pinning occurs at or in the conduction band, resulting in a contact with a nearly zero Schottky barrier height. Other examples of studies of non-alloyed ohmic contacts to n-type III/V semiconductors are T. Nittono et al., Japanese Journal of Applied Physics, Vol. 25 (10), pp. L865-L867, and W. C. Dautremont-Smith et al., Journal of Vacuum Science and Technology, B2 (4), pp. 620-625.
Lasers and light emitting diodes (LEDs) that use non-alloyed ohmic contacts to the p-electrode are known. In particular, O. Ueda et al., Journal of Applied Physics, Vol. 54 (11), pp. 6732-6739, have studied InP-based LEDs with Ti/Pt/Au non-alloyed ohmic p-contact. A. R. Goodwin et al., Journal of Lightwave Technology, Vol. 6 (9), pp. 1424-1434, report on the design and realization of InP-based lasers that use a Ti/Pt/Au non-alloyed ohmic p-side contact, together with an alloyed n-side contact.
Sputtered Ni-P was shown to form an ohmic contact to n-type InP as well as p-type InGaAs, and also to act as a diffusion barrier. Specific contact resistances of 3.times.10.sup.-6 .OMEGA..multidot.cm.sup.2 and 2.times.10.sup.-5 .OMEGA..multidot.cm.sup.2 were observed for these n-type InP and p-type InGaAs contacts, respectively. See A. Applebaum et al., IEEE Transactions on Electron Devices, Vol. ED-34 (5), pp. 1026-1031. See also U.S. patent application Ser. No. 878,077, co-assigned with this, and incorporated herein by reference. As will be recognized by those skilled in the art, NiP-deposition is not a conventional process step in III/V-device manufacture. Furthermore, the above cited specific contact resistances are relatively high. Especially the p-side specific contact resistance is too high for some important III/V-based devices which are intended to operate at high current density, e.g., fast lasers, optical amplifiers, LEDs and transistors.
In view of the commercial importance of III/V semiconductor devices such as lasers and LEDs, a method of producing such devices that avoids the shortcomings associated with prior art contacts would be highly desirable. In particular, it would be highly desirable to have available a method that has fewer processing steps, especially fewer steps subsequent to the thinning of the wafer, that is less prone to operator error due to the absence of bilateral symmetry of the metallization, and that results in thermally stable, adherent, very low specific contact resistance ohmic contacts to both n- and p-type regions of the semiconductor device. This application discloses such a method.